Simulations in Chemical and Condensed Matter Physics
This group studies a variety of complex many body systems providing insight at many length and time scales into the collective phenomena of interest. A current focus is on abstracted models of interacting polymer systems far from equilibrium in which they are accumulating data on the statisitcal distribution of chemical morphologies and dynamics. These models are intended to provide better understanding of how dynamic metastable states involving large molecules can emerge from a starting configuration of small molecules as is believed to have occurred in the origin of life. The researchers have studied a "well mixed reactor" version of such a model and are currently studying an extension in which spatial heterogeneity and diffusion can occur. Studies of the detailed morphological and dynamic character of the well mixed model also continue. Simulations using more chemically realistic descriptions of the atomic level are also planned.
A second focus is on the behavior of oxide water interfaces using in-house self consistent tightbinding codes. There is tremendous current interest in oxides as electrodes in a variety of technologies using aqueous electrolytes including fuel cells, batteries and electrolysers. Water-oxide interfaces are also a key component in corroding metal surfaces so such studies are also relevant to attempts to understand and inhibit corrosion. In one project the researchers are simulating at titania water interfaces with particular emphasis on new methods for calculating surface energies to understand the propensity of titania water interfaces to dissociate water. This project is a collaboration with former student Patrick Schelling, now an associate professor at the University of Central Florida, and his students. A second project in this category is a simulation of the magnetite water interface. In addition to its obvious corrosion relevance, the study is intended to provide better understanding of the mechanisms of water dissociation at a magnetite water interface, as occurs in experiments which the group is doing on the use of magnetite electrodes in electrolysers for production of gaseous hydrogen as an energy storage medium. This project is a collaboration with Professor Melissa Eblen of the Carleton College Physics Department, the Natural Resources Research Institute in Duluth, and high school physics teacher Jon Huber, who worked with the Halley Group in the Research Experiences for Teachers program at Minnesota in summer 2015 and continues experiments with students in Burnsville. In a third related project the researchers are beginning molecular dynamics simulations of membrane proteins in water at various solid aqueous interfaces. The aim is to explore the possible use of such membrane proteins to solve certain technical problems associated with the behavior of lithium-based electrodes in water in applications to batteries. This is a preliminary exploratory collaboration with Jonathan Sachs of the Department of Biomedical Engineering.
Thirdly the group simulates quantum fluid phenomena. A current emphasis is on condensate mediated transmission using Diffusion Monte Carlo methods to obtain informaton about excited scattering states in the strongly interacting helium four superfluid. The methods are unique and were developed in this group. They have published results in Physical Review B using a guiding wave function which did not conserve particle current. The current project is to use an improved guiding wave function which removes that defect. The project is relevant to an experiment they proposed more than a decade ago and which has been tried in various laboratories, still without a definitive result, to observe the condensate mediated transmission effect. Another quantum fluids project, carried on at a low intensity level, is the exploration of effects of disorder, and in particular of disorder induced pairing, in superconductors.
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Modeling Variability in the Earth System: From Rainfall, to River Networks, to Landscape Processes
This research group focuses on a wide range of environmental problems including inverse problems for precipitation estimation from space, stochastic theories of water/sediment transport and landscape evolution, river delta network topology and dynamics for vulnerability assessment, and river morphodynamics from satellite imagery.
Current work focuses on new and innovative formalisms for inverse estimation problems (downscaling, data fusion, retrieval, and data assimilation) of precipitation using multi-sensor, multi-scale measurements from space. New methodologies for quantifying the stochastic nature of bedload sediment transport using multi-scale analysis and dynamical system theory are studied as well. The researchers seek to understand the relations between near-bed turbulence, riverbed morphodynamics, and sediment transport using experimental, theoretical, and numerical approaches. Landscape reorganization under climate change is also studied using both controlled laboratory experiments and conceptual modeling. Dynamical frameworks for the analysis of river meandering dynamics, inferring process from form, and quantifying response to perturbations are further studied in the group. They explore quantitative frameworks for studying river delta topology and dynamics based on graph-theoretic approaches, where deltaic systems are represented by rooted directed acyclic graphs. They also work on developing metrics that capture unique physical, topological, and dynamical aspects of delta networks with the ultimate objective that deltas can be compared and contrasted and also analyzed for relative vulnerability (or resilience) to change. At the basin scale, the researchers investigate network-based frameworks for identifying potential synchronizations and amplifications of sediment delivery to basin outlets and also for identifying hotspots of fluvial geomorphic change based on dynamic connectivity. They also analyze the transport and mixing of water particles traveling in a river basin based on a stochastic Lagrangian formulation of transport to evaluate the residence time distributions, which are fundamental catchment descriptors blending key information about storage, geochemistry, flow pathways, and sources of water into a coherent mathematical framework.
The group has recently begun a campaign to map and analyze river morphodynamics in South America using Landsat imagery. In order to map migration rates for thousands of kilometers of major rivers, they have developed a nearly-automated process that extracts binary water masks from tropical rivers via an in-house supervised classifier that exploits all bands available from Landsat. A toolbox has been developed to process these binary masks to extract river banklines, centerlines, and widths at the pixel (30 m) scale to quantify migration dynamics within the most active channels in the world at an annual timescale for more than three decades. Tools for geomorphologic analysis have been developed that compute migration rates, analyze in-channel bar morphologies, and automatically delineate individual bends through time. These tools are scalable and parallelizable; a proof-of-concept was successfully performed on 1,600 km of the Ucayali River (roughly 50 Landsat scenes). Expanding this study to the entire Amazon region will require computationally intensive processing, hence having access to powerful computational resources and parallel computational capacity is essential for the efficient implementation of these researches.
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